An apparatus has been developed in which bacterial growth can be measured very precisely over short intervals of time. Its precision is presented and used to assess the constancy of growth in batch culture. Under certain conditions, i.e., Luria broth or 0.2% glucose-M9 medium at very low cell densities, the specific growth rate of Escherichia coli appeared to be constant within the measurement limits of the method. In succinate minimal medium, the growth rate increased gradually over several days and never became constant. With nutrient broth and with Luria broth, growth slowed progressively at moderate cell densities within the range considered to be in the logarithmic phase of growth. In addition, temporary slowdown in growth rate occurred in these two complex media at characteristic cell densities. These gradual increases in succinate minimal medium and temporary slowdowns in the complex media would be undetectable without precise measurements and may have been a source of variability in many bacteriological studies.